The present invention relates generally to a screening media for dewatering and the like and is more particularly concerned with a new and improved screen panel or screen deck construction and dewatering technique particularly adapted for use in dewatering and sizing slurries containing solids of fine particle size, generally 1/8 inch down to about 325 mesh, but not limited thereto.
In the processing and handling of aggregate materials such as sand, gravel or crushed stone, as well as in related industries such as the coal, slag, iron ore, phosphate, potash, primary metal and related chemical industries, it is necessary to utilize relatively large quantities of water or other liquids in conjunction with or as a dispersing medium for finely sized solid particles produced in the respective grading, concentration or other process. At some point in the process, it is usually necessary to subsequently effect a separation or dewatering of these fine solid materials.
One method of dewatering fine granular material prior to disposing of the waste water or other liquid has been to subject the mixture to suitable dewatering devices. The most widely employed method for dewatering in the mineral aggregates industry is an inclined screw dehydrator which slowly moves the solid material up the incline of the screw thread out of a feed basin permitting back flow of the water to waste. However, such equipment has limited water handling capacity and is plagued by the loss of valuable fine sands which are carried away in the back flow. Also, such equipment consumes an appreciable amount of energy, typically from 20 to 40 horsepower per unit, and in some cases more.
Another technique involves the use of centrifugal force to remove the free moisture. However, the high cost, high power consumption, and wear characteristics associated with such centrifuge apparatus have prevented the wide use of that technique on a commercial basis for handling abrasive materials. Similarly, the use of pressure or vacuum filters has not been commercially attractive, particularly for the sand and gravel and crushed stone industries.
Another technique sometimes employed for dewatering fine particle slurries is the use of conventional vibrating dewatering screens. The decks of these screens have taken the form of finely woven wire cloth or a plurality of parallel steel rods or strips separated by small gaps of predetermined gauge. These rod decks are generally constructed of stainless steel wedge shaped members.
When handling slurries containing particles below a screen mesh size of about 1/8 inch or 10 mesh, both the metal wire cloth decks and metal rod decks have a severe disadvantage as they rapidly tend to plug and blind, preventing the separation of water from the fine solids. This phenomenon occurs as a result of small particles becoming wedged in the wire cloth or rod deck openings. Such plugging and blinding problems have restricted the use of vibrating screens for fine particle dewatering, and when conventional screens are used for this purpose, their capacity is very much limited because the load must be kept "thin" and uniform to prevent plugging and subsequent blinding of the deck.
Also such wire cloth and metal rod decks exhibit the undesirable characteristic of rapidly increasing the size of the aperture or opening between the wires or rods as the top surface of the deck wears away from the abrasive action of the solids being dewatered. This condition causes valuable material that should be recovered as a dewatered fines product to pass through the deck openings enlarged by wear and be lost in the waste effluent. Attempts to cap the metal rods with abrasion resistant rubber or completely replace the rods with rubber has not proven effective because of the difficulty in maintaining very fine openings that must be used for such dewatering service. Perforated all-rubber or rubber-clad steel screen decks are commonly used by the mining, sand, gravel and crushed stone industry, but these are not offered with openings under about 1/8 inch or about 10 mesh and therefore are not used for dewatering or sizing fine granular materials.
It will also be appreciated that fine mesh wire cloth and metal rod decks when used for dewatering fine granular slurries have the additional economic disadvantage of having a relatively short life because they rapidly wear as the result of abrasion and the corrosive effect of atmosphere or the chemically active material being handled. Metal fatigue and cracking is also a common problem.
In U.S. Pat. No. 3,194,397 there is disclosed a perforated rubber panel that is clamped at opposite ends and stretched in a direction transverse to the elongated slots therein to adjust the width of the slots. The panel moves vertically in trampoline fashion during vibration since it is secured to its frame at only the ends of the panel. This vertical motion causes the slots to expand and contract thereby providing a self-cleaning effect. When particles become lodged, the panel is further tensioned to allow them to fall through. Reinforcing elements are also provided parallel to the slots to limit tensioning in one direction only while dampening the trampoline action of the panel without interferring with the adjustability of the slots. Such vertical or trampoline action is believed to be impractical in commercial operations and places severe stress on the entire screen deck.
As can be appreciated, the industry is constantly seeking new and improved methods for dewatering large quantities of fine solid slurries.
Accordingly, it is a principal object of the present invention to provide a new and improved dewatering screen deck and dewatering method that obviates many of the deficiencies and difficulties encountered in prior dewatering techniques by utilizing a unique dewatering action that substantially increases both the capacity and efficiency of the dewatering operation at lower cost. Included in this object is the provision for a vibrating dewatering screen deck panel that not only oscillates at the predetermined frequency of the driving vibrator, but additionally provides a random array of independently pulsating dewatering diaphrams that rapidly and efficiently dewater or separate the liquid phase from a slurry containing solids of fine particle size.
Another object of the present invention is to provide a new and improved dewatering screen deck that is characterized by a self-cleaning action and the absence of undesirable plugging and blinding of the fine solids without the necessity for constantly adjusting the size of the openings in the deck.
Another object of the present invention is to provide a new and improved dewatering technique adapted for efficient high capacity operation in the removal of free moisture from a substantially thicker or deeper slurry feed by combining a planar pulsating action at multiple locations throughout the screen with simultaneous resonating vibration of the entire screen to very rapidly and smoothly "cake" the solid component including the "fines" and move the resultant particulate cake across the screen without substantial loss of the extremely fine particles therefrom, or plugging.
Another object of the present invention is to provide a new and improved nonadjustable dewatering screen deck operative in the manner described while at the same time obviating the deficiencies of the finely woven wire cloth and steel rod decks in that wear will not effect change in the screen openings and the deck operates in a self-cleaning manner to prevent plugging and blinding.
Another object of the present invention is to provide a method of dewatering fine granular slurries which requires significantly lower power consumption than that required for conventionally used apparatus such as screw dehydrators, centrifuges, vacuum filters and the like.
Still another object is the provision for a unique long wearing, nonadjustable, nonblinding screen deck that also can be used for sizing fine particulate material of less than about 1/8 inch.
Other objects will be in part obvious and in part pointed out in more detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a new and improved screen deck assembly suited to vibratory dewatering of fluid slurries containing fine particulate solid material. The screen deck assembly is comprised of individual units consisting of a rigid frame member and a substantially planar screen panel fixedly mounted within the frame. Each panel is tensioned in both planar directions before being secured to the frame and consists of a plurality of pulsator portions and integral stabilizer portions bordering the pulsator portions. The tensioning in both planar directions draws the panel into a substantially rigid or taut condition and the stabilizer portions of the panel are positively and immovably secured to the rigid frame member. The screen panel within the frame is a unitary one piece member composed of an elastomeric material throughout its entire extent. The pulsator portions of the elastomeric panel are comprised of a multitude of substantially identical diaphram pulsator elements arranged in intimate, aligned and confronting relationship so as to present a smooth top screening surface. The pulsator elements extend through full depth of the panel and define a column of spaced parallel slits between individual abutting elements to permit limited random pulsating motion of the flexible elements within the plane of the panel in addition to the vibratory motion of the entire screen assembly.
An additional benefit of the present invention is the provision for a unique and exceptionally efficient dewatering process which utilizes the aforementioned dewatering screen deck assembly fitted to a vibrating screen mechanism operating at a fixed frequency and fixed amplitude sufficient to effect the desired additional random frequency pulsating motion of the pulsator elements independently of the induced, fixed vibratory motion of the vibrator mechanism. A fluid slurry of fine particulate solid material fed to the vibrating screen deck passes across the deck in a direction substantially normal to the elongated slits between the elements so that the pulsating motion of the elements within the plane of the deck effectively draws the fluid from the slurry while simultaneously allowing the solid particulate material therein to form a coherent mass for vibratory conveyance along the smooth and bi-axially taut top surface of the screen assembly without substantial loss of the fine particulate material.
Still another benefit of the present invention is the provision for a process whereby very accurate classification by size of fine particles may be accomplished. Such process involves feeding to the deck of the invention a dilute water and solids slurry over the major length of the deck. In such mode of operation the water rapidly passes through the openings in the panel carrying with it particles of a size finer than the openings in the panel.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawing which set forth an illustrative embodiment and are indicative of the way in which the principles of the invention are employed.